Companies and individuals rely on having a consistent supply of power to electronic devices more than ever before. Without power, companies may be unable to manufacture goods, or to operate at all, such as if the company is in the business of supplying information over the Internet. Without power, businesses and individuals may be completely incapacitated regarding critical activities, such as designing products, making goods, providing services, and transacting personal finances (e.g., filing tax returns, and paying bills). Uninterruptible power supplies (UPSs) are often used to provide backup power in case of a power outage. UPSs are commonly used on computing equipment to guard against data being lost due to a power outage before the data are saved. UPSs used with computing equipment also help to guard against a loss in service by providers of information over the Internet, such as by servers, e.g., hosting web pages.
Online UPS systems typically contain a boost stage power factor correction (PFC) front-end converter 502 and an inverter stage rear end 504 as shown in FIG. 1 for the UPS system 500. The inputs to the front-end converter are a 60/50 HZ AC supply 506 and Battery DC supply 508. The UPS 500 works in two modes of operation based on the input voltage. When the input AC voltage is within an acceptable range for the boost converter 502 to operate on the AC supply voltage, the UPS 500 works in an online mode. In this mode, the front-end boost converter 502 takes input power from AC supply 506 and converters the voltage to two DC voltages and provides these voltages to two DC busses 510, 512, with a positive DC bus voltage (+DC) and a negative dc bus voltage (−DC), respectively. When the input AC voltage is not available or not within the acceptable range, the UPS works in an on-battery mode. In this mode, the front-end boost converter 502 takes DC input power from the battery 508 and produces positive and negative DC bus voltages and delivers these voltages to the respective busses 510, 512.
A central control system (controller, not shown) in the UPS system 500 monitors the input AC voltage and controls transfers between the two different modes. Traditionally, relays have been used to transfer the front-end boost converter inputs from AC supply to DC supply and vice versa. Recently, Silicone Controlled Rectifiers (SCRs) have been used for this purpose.
Two bulk capacitors 514, 516 are provided between the DC busses 510, 512. The capacitors 514, 516 are part, of the front end converter 502, but shown outside the converter 502 for illustrative purposes. The capacitors 514, 516 provide energy to a load through the inverter 504 during transfers between different modes of the UPS 500 to help ensure transfers without significant voltage drops to the load.
The inverter 504 is a DC-AC converter that takes input from the positive and negative DC bus voltages and produces an AC voltage at the output. The inverter 504 in typical online UPS systems comprises two buck converters that are controlled by pulse width modulation (PWM) controllers to provide a desired sine wave output.
Referring to FIG. 2, with further reference to FIG. 1, a positive buck converter 522 converts DC voltage from the +DC bus 510 to AC voltage during positive half cycles of the output voltage and a negative buck converter 524 converts DC voltage from the −DC bus 512 to AC voltage during negative half cycles of the output voltage. The outputs of both of the buck converters 522, 524 are combined to get a full cycle of AC voltage. In other words, load power is supplied from the +DC bus 510 during positive half cycles of the output (load) voltage and load power is supplied from the −DC bus 512 during negative half cycles of the output voltage.
Power Factor Correction of Online UPS Systems
There are two modes of front-end boost operation in online UPS systems, online mode and on-battery mode.
Online Mode
Referring to FIG. 3, with further reference to FIGS. 1-2, in online mode the front-end converter 502 uses a positive boost converter 526 and a negative boost converter 528. The front-end converter 502 takes input from the AC supply 506 and outputs two DC voltages. The front-end converter 502 works as a PFC converter while it is working from AC input voltage. The positive boost converter 526 converts positive half cycles of AC input voltage to DC voltage during the positive half cycles of the input voltage. This positive output is given to the +DC bus capacitor 514. The negative boost converter 528 converts negative half cycles of AC input voltage to DC voltage during the negative half cycles of the input voltage. This negative output is given to the −DC bus capacitor 516.
Even though the front-end converter 502 uses two converters 526, 528 to supply power to the two DC buses 510, 512, some of the components (e.g., inductors and current transformers) can be shared so these can be common components for both positive and negative boost converters 526, 528.
FIGS. 4-6 show three circuits 550, 560, and 570 for implementing PFC from an AC supply. The circuits 550, 560, 570 contain positive converters 552, 562, 572 and negative converters 554, 564, 574. The circuit in Black is used in both positive and negative half cycles. These methods are well discussed in the literature. The number of components are not the same for all three topologies. The selection of the topology (circuit) depends on several factors such as power level, control architecture, etc. Of the three circuits 550, 560, 570, the circuit 550 offers several advantages at lower power levels such, as high, efficiency, low cost, simple control implementation and lower part count.
On-Battery Mode
Referring to FIG. 7, with further reference to FIG. 1, in the on-battery mode of operation, the front-end converter 502 takes input power from the battery 508 as the voltage source and delivers power to both the positive and negative DC buses 510, 512. The battery 508 can be connected in different configurations, such as positive non-floating, negative non-floating, or floating. The battery 508 provides positive non-floating voltage when the battery's negative terminal is connected to the neutral and provides negative non-floating voltage when the battery's positive terminal is connected to the neutral. In floating configuration, neither of the battery's terminals is net connected to neutral. The non-floating battery (one terminal of the battery is connected to neutral) simplifies battery voltage sensing and also simplifies the charger control.
Power conversion implementation techniques are different for floating and non-floating batteries in PFC. Known implementations are using boost and buck-boost converters for non-floating battery systems as shown in FIG. 7, and using twin boost converters for floating battery systems as shown in FIG. 9.
Boost and Buck-Boost Converters
As discussed above, one DC voltage from the battery 508 is used to derive two DC output voltages with different polarities. A boost converter 580 is used to boost the battery voltage to the DC bus voltage with the same polarity as the battery 508. The boost converter operation is discussed in above in online mode of operation. A buck-boost converter 590 may be used to derive a DC voltage from the battery 508 with a polarity opposite that of the battery 508.
Referring to FIG. 8, the buck-boost converter 590 includes a buck portion 592 and a boost portion 594. The buck portion 592 includes a switch 596 and an inductor 598, and, the boost portion 594 includes the inductor 598, and a diode 600. When the switch 596 is ON (closed), current flowing through the inductor 598 increases and stores energy. When the switch 596 is OFF (open), the stored energy in the inductor 598 is transferred to the capacitor 516. Thus, while the switch 598 is ON the current path is through the battery 508, the switch 596, and the inductor 598, and while the switch 596 is OFF the current path is through the inductor 598, the capacitor 516, and the diode 600. For a battery with a voltage between 120 VDC and 240 VDC and an output bus voltage of +400 volts, the switch 596 should be rated at 1200V as the switch 596 switches the battery voltage plus the +DC bus voltage. Also, the diode 600 should be rated at 1200V.
The total front-end converter 502 uses two converters (boost 580 and buck-boost 590) to transfer energy from the battery 508 to the positive and negative DC buses 510, 512. The converters 580, 590 are separate converters and do not share components during on-battery operation. Because the two converters 580, 590 are separate and operate concurrently, for improved efficiency, in on-battery operation, single inductor solutions shown in FIGS. 4 and 5 can not be implemented.
Twin Boost Converters
Another approach to output both positive and negative voltages from a single battery is by using a floating battery and a twin boost converters configuration 610 as shown in FIG. 9. In this configuration, neither of the battery terminals is connected to the neutral, unlike the buck-boost approach. Operation of the twin boost converter is discussed in U.S. Pat. No. 5,654,591.
Three-Phase Applications
Prior three-phase front-end topologies typically have used fully decoupled PFCs, while some have used partially-decoupled PFCs at lower power levels because of a fewer CTs and better utilization of silicon and magnetics. For example, a three-phase, partially-decoupled PFC is discussed in U.S. Pat. No. 7,005,759, in which four inductors are used to implement three-phase front-end converter in an online system. Referring to FIG. 10, which is FIG. 3 in U.S. Pat. No. 7,005,759, a three-phase front-end converter 620 includes a switch So that is open during the online operation so that the PFC can work from input AC through inductors La, Lb, and Lc and diodes D1 to D6. The inductors La, Lb, Lc are boost inductors for the three-phase PFC. A description of PFC operation can be found in “Quasi-Soft-Switching Partly Decoupled Threephase PFC. With Approximate Unity Power Factor” by David M. XU C. Yang J. H. Kong Zhaoming. Qian (IEEE, 1998). During the on-battery operation, the switch So is closed and the battery supplies the power to the dc buses. The inductor L is used as boost inductor in the on-battery mode of operation. CTs are not shown but would be disposed between La and D1, Lb and D2, Lc and D3, and between the battery and L.